mtd: mtdpart: use DEVICE_ATTR_RO() helper macro

Use DEVICE_ATTR_RO() helper macro instead of plain DEVICE_ATTR(), which
makes the code a bit shorter and easier to read.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210603123041.12036-1-thunder.leizhen@huawei.com